DE102022110114A1 - Redox flow cell - Google Patents

Abstract

The invention relates to a redox flow cell (8), comprising at least one electrode (1, 1', 1a, 1b) and an electrolyte in contact with the electrode (1, 1', 1a, 1b), the electrode (1, 1 ', 1a, 1b) comprises a substrate (2) which is formed from at least one electrode material in the form of a metal sheet (2a) and/or an expanded metal grid (2b). The electrode material is formed from a copper-bismuth alloy (CuBi), with the substrate (2) being covered by a polymeric cover layer (5) containing electrically conductive particles, at least in its contact area with the electrolyte.

Description

The invention relates to a redox flow cell, in particular a redox flow battery, comprising at least one electrode and at least one electrolyte in contact with the electrode, the electrode comprising a substrate made of at least one electrode material in the form of a metal sheet and/or a Expanded metal grid is formed.

Electrodes and redox flow cells or redox flow cells equipped with them, in particular redox flow batteries or flow batteries comprising a plurality of such cells, are well known. The redox flow battery is a storage device for electrical energy, whereby the electrical energy is stored in liquid chemical compounds or electrolytes, a so-called anolyte and a so-called catholyte. The electrolytes are located in two reaction spaces that are separated from each other by an ion exchange membrane. An ion exchange between anolyte and catholyte takes place via this membrane, releasing electrical energy. The electrical energy released is tapped via an electrode that is in contact with the anolyte and catholyte. The electrolytes are circulated in the reaction spaces using pumps and flow along the respective facing surface of the membrane. Since the electrolytes can be stored in tanks of any size, the amount of energy stored in the redox flow battery only depends on the size of the tanks used.

Flow battery systems as storage systems enable a sustainable energy supply for stationary and mobile applications using renewable energies. In order to achieve high efficiencies and power densities, the aim is to have cell structures in battery stacks that are as compact as possible. However, high power densities pose major challenges for the individual components of a battery stack.

The WO 2018/146342 A1 discloses various lignin-based electrolyte compositions for use in redox flow batteries.

The publication “A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries”, Aaron Hollas et al., Nature energy, Vol. 3, June 2018, pages 508 - 514 , describes anolytes for redox flow batteries based on aqueous “organic” electrolytes or based on aqueous electrolytes with a redox-active organic species. These are becoming increasingly important.

Currently, plate-shaped composites made of plastic and graphite are often used as corrosion-resistant substrates for electrodes of redox flow cells or redox flow batteries due to the use of strongly basic or acidic electrolytes. These substrates are usually provided with a carbon coating applied on both sides or there is a flow-through carbon felt between the membrane and the electrode. A total plate thickness of the electrode in the range of approximately 0.7 - 1.2 mm is common. Such electrodes are often held in an electrically insulating plastic frame, which involves additional costs for the frame and the assembly process. The size and manufacturing requirements of such electrodes currently stand in the way of a space-saving and, in particular, compact geometry of redox flow cells and their rational industrial production.

For technical background reference is made to the publication “Engineering aspects of the design, construction and performance of modular redox flow batteries for energy storage”, L.F. Arenas et al., Journal of Energy Storage 11 (2017), pages 119 - 153, referenced.

The DE 10 2019 121 673 A1 discloses an electrode unit for a redox flow cell comprising a metallic substrate with a coating of titanium niobium carbide and/or titanium niobium nitride. A metallic adhesion promoter layer can be provided between the substrate and the coating. Furthermore, a cover layer made of carbon or a metal carbide can be formed on the coating.

The DE 10 2005 008 082 A1 describes an electrically conductive element for an electrochemical cell comprising a metal substrate with an electrically conductive coating covered by an electrically conductive polymer-based coating. The electrically conductive coating is preferably formed by means of an ion-assisted PVD process and contains elements from groups 4, 5, 10 or 11, in particular at least one element from the group comprising titanium, zirconium, vanadium, niobium, tantalum, gold, platinum. The polymer-based coating preferably has polymer and conductive particles. The polymer-based coating is preferably applied by coating, brushing, spraying, spreading, dipping, rolling, laminating or powder coating.

It is the object of the invention to provide a redox flow cell comprising at least one electrode and at least one electrolyte, the electrode having small thickness dimensions can be produced cost-effectively and with increased corrosion resistance.

The object is achieved for the redox flow cell comprising at least one electrode and at least one electrolyte in contact with the electrode, the electrode comprising a substrate which is formed from at least one electrode material in the form of a metal sheet and/or an expanded metal grid, in that the Electrode material is formed from a copper-bismuth alloy (CuBi), the substrate being covered at least in its contact area with the electrolyte by a polymeric cover layer containing electrically conductive particles.

The use of a substrate made of a copper-bismuth alloy has proven to be particularly suitable for use in a redox flow cell. On the one hand, the electrical conductivity is high and remains stable over a long operating period of the cell in the range of more than 26 hours, as no corrosion could be detected.

The electrical conductivity of the polymeric top layer can be adjusted by selecting the electrically conductive particles. The polymeric cover layer can be designed to be liquid-tight or largely liquid-tight and is not removed or damaged by the incoming electrolyte over long periods of time.

The copper-bismuth alloy is preferably formed to contain up to 10% by weight of bismuth.

Depending on the dimensions of an electrode, it is advantageous to increase the thickness of the electrode as the area of the contact areas with the electrolyte increases in order to ensure the mechanical stability of the electrode. In principle, metal sheets and expanded metal grids with a thickness of 0.1 mm or more can be used for electrodes. However, it has proven useful if the metal sheet and the expanded metal grid are each designed with a maximum thickness of 5 mm.

In a preferred embodiment of the electrode, the metal sheet and/or the expanded metal grid has a three-dimensional profiling at least in some areas. This increases the later available contact surface of the metal sheet or expanded metal grid to an electrolyte.

The substrate can comprise only a metal sheet, only an expanded metal grid (possibly in combination with an electrically conductive support plate that cannot be penetrated by electrolyte, or a combination of metal sheet and expanded metal grid. If only one expanded metal grid is provided through which an electrolyte can flow, this can rolled up into a roll, stacked in layers on top of each other or provided in a single layer.

When combining a metal sheet with an expanded metal grid, the expanded metal grid is arranged facing the electrolyte, with preferably only a clamping between the metal sheet and the expanded metal grid. However, to simplify later assembly of the electrode in a redox flow cell, the expanded metal grid can also be attached to the metal sheet via individual welding points or glued or soldered to the metal sheet in places.

The substrate preferably has a three-dimensional profiling on one side or preferably on both sides, at least in some areas, forming a flow field. The introduction of such a flow field into a substrate is possible in a cost-effective manner by embossing or the like. Such a flow field directs the flow of the electrolyte in defined paths and is equivalent to a three-dimensional structure in the area of the surface of the substrate. It ensures a homogeneous distribution and flow of the electrolyte on and along an ion exchange membrane.

• - Kohlenstoff oder
• - einem Metallkarbid oder
• - mindestens einem Material aus der Gruppe umfassend Titan, Hafnium, Niob, Tantal, Zirkonium, oder
• - mindestens einem Material aus der Gruppe umfassend Titan, Hafnium, Niob, Tantal, Zirkonium, und weiterhin mindestens einem nicht-metallischen Element der Gruppe umfassend, N, C, F, B, O, H.

Furthermore, at least one intermediate layer is preferably present, which is applied between the substrate and the cover layer, the intermediate layer being formed from

• - carbon or
• - a metal carbide or
• - at least one material from the group comprising titanium, hafnium, niobium, tantalum, zirconium, or
• - at least one material from the group comprising titanium, hafnium, niobium, tantalum, zirconium, and further at least one non-metallic element from the group comprising N, C, F, B, O, H.

The at least one intermediate layer is intended to improve the adhesion of the cover layer to the substrate and also significantly extend the chemical stability of the electrode and its service life.

The electrically conductive particles in the polymeric cover layer are preferably formed from carbon and/or at least one metal and/or at least one metal carbide and/or at least one metal nitride and/or boron nitride. In particular, the electrically conductive particles are made of the at least one metal in the form of titanium and/or copper and/or silicon and/or aluminum nium and/or iron and/or zinc and/or tin and/or gold and/or silver.

In a preferred embodiment, the cover layer has at least one polymer in the form of polypropylene (PP), polyethylene (PE), polyamide (PA), polyvinyl chloride (PVC), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), poly (3,4) - ethylenedioxythiophene (PEDOT), polyether ether ketones (PEEK), phenazine derivatives or quinones.

The intermediate layer preferably has a layer thickness in the range from 0.1 to 500 μm.

The intermediate layer is preferably formed on the substrate using a PVD process or a combined PVD/PACVD process. It is advantageous if the intermediate layer is deposited as pore-free as possible or at least only has pores with a diameter of less than 0.1 mm in order to further reduce corrosive attack by the electrolyte on the substrate. However, the intermediate layer can also be applied using an alternative coating process, for example a CVD, PACVD or PVD process, chemical or electrochemical deposition or thermal spraying. If thin-film application processes are used, layer thicknesses of the intermediate layer are particularly preferred in the range from 200 to 1600 nm, in particular in the range from 200 to 800 nm. After the formation of the at least one intermediate layer, a heat treatment step can take place.

The intermediate layer can also be in the form of a plating, provided it is made of metal. In metal processing, plating refers to the one- or two-sided application of one or more metal layers to a different base metal. An inseparable connection is achieved through pressure and/or temperature or subsequent heat treatment (e.g. diffusion annealing). The plating can be carried out in particular by rolling thin metal foil.

In particular, it has proven useful if the electrode is formed from a metal sheet, which is provided on one or both sides with an intermediate layer which is formed by plating.

The polymeric cover layer preferably has a layer thickness D > 500 nm. The cover layer is preferably formed by a spraying process, knife coating or spin coating. After the top layer has been formed, a heat treatment step can take place.

It has proven useful if the optional at least one intermediate layer and/or the cover layer covers/covers the substrate at least on one side, preferably on both or all sides. Particularly in the area of the edges of a metal sheet, there may be uncoated areas or areas with a very small layer thickness, which, however, are usually not in contact with an electrolyte due to the separation of the electrolyte spaces and are therefore not critical. At least the optional at least one intermediate layer and the cover layer should cover the substrate in a contact area with an electrolyte of the redox flow cell, i.e. in an area that is used in direct contact with an anolyte or catholyte.

The redox flow cell preferably comprises at least two electrodes, a first reaction space for receiving a first electrolyte and a second reaction space for receiving a second electrolyte, each reaction space being in contact with one of the electrodes and the reaction spaces being separated from one another by an ion exchange membrane.

The use of an electrode with a metallic substrate, here made of a copper-bismuth alloy, enables small distances to the ion exchange membrane and thus a space-saving structure of a redox flow cell.

The electrode preferably ensures perfect separation of the reaction spaces within a redox flow cell. At the same time, such electrodes have surfaces that, in addition to the high requirements for electrochemical stability, also meet the requirements for low interfacial resistance and high catalytic activity.

In particular, flow batteries with aqueous electrolytes comprising a redox-active species on the anolyte side are preferred applications for the electrode described.

Several redox flow cells according to the invention in particular form the components of a redox flow battery. For this purpose, the redox flow cells are interconnected electrically and fluidly.

Due to the small possible thicknesses of the electrodes, small-sized redox flow batteries can be produced, which also have a low production price. To form a redox flow battery, it is preferred to use more than 10, in particular more than 50, redox flow cells electrically connected to one another.

• 1.4 M 7,8-Dihydroxyphenazin-2-sulfonsäure (kurz: DHPS) gelöst in 1 molarer Natronlauge

Anolyte that is suitable for a redox flow cell or a redox flow battery is mentioned here as an example:

• 1.4 M 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS for short) dissolved in 1 molar sodium hydroxide solution

• 0.31 M Kaliumhexacyanoferrat(II) und 0.31 M Kaliumhexacyanoferrat(III) gelöst in 2 molarer Natronlauge.

The following are examples of catholytes that are suitable for a redox flow cell or a redox flow battery:

• 0.31 M potassium hexacyanoferrate (II) and 0.31 M potassium hexacyanoferrate (III) dissolved in 2 molar sodium hydroxide solution.

Electrolyte combinations with aqueous electrolytes with a redox-active organic species on the anolyte side are preferably used to form a redox flow cell or a redox flow battery.

• 1 eine Elektrode umfassend ein Substrat und eine Deckschicht in der Draufsicht auf die Substratebene,
• 2 einen Querschnitt durch die Elektrode gemäße 1,
• 3 einen Querschnitt durch eine Elektrode mit einer Profilierung,
• 4 einen Querschnitt durch eine Elektrode umfassend ein Substrat aus einem Metallblech und einem Streckmetallgitter,
• 5 eine Elektrode mit einem Flussfeld, und
• 6 eine Redox-Flusszelle beziehungsweise eine Redox-Flow-Batterie mit einer Redox-Flusszelle.

The 1 until 6 show example electrodes for a redox flow cell and a redox flow cell or a redox flow battery. So shows

• 1 an electrode comprising a substrate and a cover layer in a top view of the substrate plane,
• 2 a cross section through the electrode 1 ,
• 3 a cross section through an electrode with a profiling,
• 4 a cross section through an electrode comprising a substrate made of a metal sheet and an expanded metal grid,
• 5 an electrode with a flow field, and
• 6 a redox flow cell or a redox flow battery with a redox flow cell.

1 shows an electrode 1 comprising a substrate 2 in a top view of the substrate plane. The substrate 2 is here formed from a metal sheet 2a with a thickness of less than 0.5 mm. The metal sheet 2a is formed from a copper-bismuth alloy with a bismuth content of 5% by weight. On the substrate 2 there is a polymeric cover layer 5 containing electrically conductive particles made of titanium in a polymeric matrix made of PVC.

2 shows a cross section through the electrode 1 according to 1 comprising the substrate 2 in the form of the metal sheet 2a made of the copper-bismuth alloy, which has the polymeric cover layer 5 on both sides. The cover layer 5 can also only be applied to one side of the metal sheet 2a, with the cover layer 5 intended to cover the substrate 2 at least in a contact area with an electrolyte of the redox flow cell 8 (see 6 ). The polymeric cover layer 5 here has a layer thickness D of 700 nm.

3 shows a cross section through an electrode 1 comprising a substrate 2 in the form of a metal sheet 2a made of a copper-bismuth alloy. The metal sheet 2a has a three-dimensional profiling 4, which increases the later contact area of the metal sheet 2a to an electrolyte. The substrate 2 is covered on all sides with an intermediate layer 3 made of titanium (layer thickness in the range from 200 to 800 nm) and then a polymeric cover layer 5 (layer thickness 1 μm), which are not shown in detail here due to the small layer thicknesses. The cover layer 5 here has particles made of niobium

4 shows a cross section through an electrode 1' comprising a substrate 2, which comprises a metal sheet 2a and an expanded metal grid 2b made of a copper-bismuth layer. The metal sheet 2a and the expanded metal grid 2b are covered by a polymeric cover layer 5.

5 shows an electrode 1 in a three-dimensional view comprising a substrate 2 in the form of a metal sheet 2a with a profiling 4 that forms a flow field 7 and with openings 6 for an inflow and outflow of electrolyte. In the substrate 2, the profiling 4 is present on both sides to form a flow field 7, so that a three-dimensional structuring of the surface of the electrode 1 results, which is to be flowed by an electrolyte in a redox flow cell. The substrate 2 is covered on both sides with an intermediate layer 3 and the intermediate layer 3 with the polymeric cover layer 5.

6 shows a redox flow cell 8 or a redox flow battery with a redox flow cell 8. The redox flow cell 8 comprises two electrodes 1a, 1b, a first reaction space 10a and a second reaction space 10b, each reaction space 10a, 10b being in contact with one of the electrodes 1a, 1b. The reaction spaces 10a, 10b are separated from one another by an ion exchange membrane 9. A liquid first electrolyte, here anolyte 11a, is pumped from a tank 13a via a pump 12a into the first reaction space 10a and passed between the electrode 1a and the ion exchange membrane 9. A liquid second electrolyte, here catholyte 11b, is pumped from a tank 13b via a pump 12b into the second reaction space 10b and passed between the electrode 1b and the ion exchange membrane 9. An ion exchange takes place across the ion exchange membrane 9, with electrical energy being released at the electrodes 1a, 1b due to the redox reaction.

Reference symbol list

1, 1', 1a, 1b

electrode

2

Substrate

2a

metal sheet

2 B

Expanded metal grid

3

Interlayer

4

Profiling

5

top layer

6

Openings for inflow and outflow of electrolyte

7

River field

8th

Redox flow cell or redox flow battery

9

Polymer electrolyte membrane

10a

first reaction space

10b

second reaction space

11a

Anolyte

11b

catholyte

12a, 12b

pump

13a, 13b

tank

D

Thickness of the top layer

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2018146342 A1 [0004]
• DE 102019121673 A1 [0008]
• DE 102005008082 A1 [0009]

Non-patent literature cited

• Aaron Hollas et al., Nature energy, Vol. 3, June 2018, pages 508 - 514 [0005]

Claims (10)

Redox flow cell (8), comprising at least one electrode (1, 1', 1a, 1b) and an electrolyte in contact with the electrode (1, 1', 1a, 1b), the electrode (1, 1', 1a, 1b) comprises a substrate (2) which is formed from at least one electrode material in the form of a metal sheet (2a) and/or an expanded metal grid (2b), characterized in that the electrode material is formed from a copper-bismuth alloy (CuBi). , wherein the substrate (2) is covered at least in its contact area with the electrolyte by a polymeric cover layer (5) containing electrically conductive particles. Redox flow cell (8). Claim 1 , characterized in that the copper-bismuth alloy is formed comprising up to 10% by weight of bismuth. Redox flow cell (8) according to one of the Claims 1 or 2 , characterized in that the metal sheet (2a) and/or the expanded metal grid (2b) is each designed with a maximum thickness of 5 mm. Redox flow cell (8) according to one of the Claims 1 until 3 , characterized in that the metal sheet (2a) and/or the expanded metal grid (2b) has a three-dimensional profiling (4) at least in some areas. Redox flow cell (8) according to one of the Claims 1 until 4 , characterized in that it further comprises at least one intermediate layer (3) which is applied between the substrate (2) and the cover layer (5), the intermediate layer (3) being formed from - carbon or - a metal carbide or - at least one Material from the group comprising titanium, hafnium, niobium, tantalum, zirconium, or - at least one material from the group comprising titanium, hafnium, niobium, tantalum, zirconium, and further at least one non-metallic element from the group comprising, N, C, F, B, O, H Redox flow cell (8) according to one of the Claims 1 until 5 , characterized in that the electrically conductive particles in the cover layer (5) are formed from carbon and/or at least one metal and/or at least one metal carbide and/or at least one metal nitride and/or boron nitride. Redox flow cell (8). Claim 6 , characterized in that the electrically conductive particles consist of the at least one metal in the form of titanium and/or copper and/or silicon and/or aluminum and/or iron and/or zinc and/or tin and/or gold and/or silver present. Redox flow cell (8) according to one of the Claims 1 until 6 , characterized in that the cover layer (5) has at least one polymer in the form of polypropylene (PP), polyethylene (PE), polyamide (PA), polyvinyl chloride (PVC), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), poly (3, 4)-ethylenedioxythiophene (PEDOT), polyether ether ketones (PEEK), phenazine derivatives or quinones. Redox flow cell (8) according to one of the Claims 1 until 8th , characterized in that the intermediate layer (3) has a layer thickness in the range from 0.1 to 500 µm and/or that the cover layer (5) has a layer thickness (D) > 500 nm. Redox flow cell (8) according to one of the Claims 1 until 9 , comprising at least two electrodes (1a, 1b), a first reaction space (10a) for receiving a first electrolyte and a second reaction space (10b) for receiving a second electrolyte, each reaction space (10a, 10b) being in contact with one of the electrodes ( 1a, 1b) and the reaction spaces (10a, 10b) are separated from one another by an ion exchange membrane (9).

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